1. Field of the Invention
The present invention relates to an alternating current to direct current (AC-DC) converter that receives an AC power supply and that outputs a DC voltage, and particularly, to a power-factor correction (PFC) converter for correcting a power factor.
2. Description of the Related Art
In Japan and Europe, for example, harmonic current control that is classified in accordance with an application or input power is performed. In order to respond to such control, a circuit called a PFC converter is added to a power supply of general home appliances subjected to harmonic current control, whereby measures have been taken to suppress harmonic currents.
In a typical switching power supply device using a commercial AC power supply as an input power supply, the commercial AC power supply is rectified and smoothed so as to be converted to a DC voltage, and switching by a DC-DC converter is performed on the DC voltage. Therefore, an input current is discontinuous and is significantly distorted as compared to a sinusoidal wave. This causes a harmonic current.
For the purpose of suppressing the harmonic current, a PFC converter is provided in a stage after a full-wave rectifier circuit and before a smoothing circuit that includes a smoothing capacitor.
This PFC converter, which includes a chopper circuit, operates so that an input current waveform is similar to an input voltage waveform, i.e., so that the waveforms are sinusoidal waveforms having the same phase. Accordingly, a harmonic current is suppressed to a certain level or lower.
An example configuration of the PFC converter disclosed in Japanese Unexamined Patent Application Publication No. 2004-282958 will be described with reference to FIG. 1.
In the power-factor correction circuit illustrated in FIG. 1, a series circuit including a step-up reactor L1, a switching element Q1 defined by a MOSFET, and a current detecting resistor R is connected to both output terminals of a diode bridge B1 that rectifies an AC power supply voltage of an AC input power supply Vac1. A series circuit including a diode D1 and a smoothing capacitor C1 is connected to both ends of the switching element Q1, and a load RL is connected to both ends of the smoothing capacitor C1. The switching element Q1 is turned on/off under pulse width modulation (PWM) control by a control circuit 10. The current detecting resistor R detects an input current flowing through the diode bridge B1.
The control circuit 10 includes an error amplifier 111, a multiplier 112, an error amplifier 113, a voltage controlled oscillator (VCO) 115, and a PWM comparator 116.
The error amplifier 111 calculates an error between the voltage of the smoothing capacitor C1 and a reference voltage E1. The multiplier 112 multiplies an error voltage signal by a voltage rectified by the diode bridge B1. The error amplifier 113 generates an error between a multiplication result generated by the multiplier 112 and a current signal flowing through the diode bridge B1 and outputs the error to the PWM comparator 116.
The VCO 115 generates a triangular-wave signal of a frequency according to a voltage value of a rectified AC power supply voltage.
In the PWM comparator 116, a triangular-wave signal from the VCO 115 is input to a minus terminal, and a signal from the error amplifier 113 is input to a plus terminal. That is, the PWM comparator 116 applies a duty pulse according to a current flowing through the diode bridge B1 and an output voltage to the switching element Q1. This duty pulse is a pulse-width control signal that continuously compensates for fluctuations of an AC power supply voltage and a DC load voltage in constant cycles. With this configuration, control is performed so that the waveform of an AC power supply current matches the waveform of an AC power supply voltage, whereby the power factor is corrected.
On the other hand, a digitally-controlled PFC converter is disclosed in Japanese Unexamined Patent Application Publication No. 7-177746.
In the case of digital control, a current flowing through an inductor is detected, and switching is performed on a switching element under PWM control according to the current value.
As described above, in the PFC converter, it is necessary to detect a current flowing through an inductor (hereinafter “inductor current”) in order to cause an input current waveform to be similar to an input voltage waveform. For this purpose, the following methods may be typically used.
(a) Directly detect a current flowing through an inductor.
(b) Detect a current flowing through a switching element and treat the current as being equivalent to an inductor current.
(c) Detect a current flowing through a diode provided on an output side and treat the current as being equivalent to an inductor current.
In addition, the following methods may be used to provide a current detecting means.
(1) Insert a current detecting resistor in series into a current path and detect a decreased voltage at both ends of the resistor.
(2) Insert a current transformer into a current path or perform detection using a current transformer in which an inductor is on the primary side.
(3) Provide a hole sensor in a current path and detect an output voltage thereof.
In the above-described method (1) of using a current detecting resistor, power consumption in the current detecting resistor itself is loss, which is a problem in terms of reducing loss. In the method (2) of using a current transformer, a DC component of a current that is to be detected is cut, and thus, only an AC component of the current is detected and the DC component of the current (DC offset) cannot be detected. If each of the currents in the above-described methods (b) and (c) is detected using a current transformer and signals are combined, an inductor current can be detected. However, in this case, two current transformers are required. In the method (3) of using a hole sensor, the problems that may occur in the methods (1) and (2) do not arise, but the overall cost increases because the sensor is expensive.